Islamic State billboards are seen along a street in Raqqa Thomson Reuters AMMAN (Reuters) - An alliance of Kurdish-led armed groups fighting Islamic State in northern Syria said they had launched an offensive on Tuesday to seize countryside north of the militants' de facto capital Raqqa, their spokesman said.

Talal Silo, spokesman for the U.S.-backed Syria Democratic Forces (SDF), whose main component is the powerful Kurdish YPG militia, told Reuters that the campaign at this stage did not include an assault on Raqqa itself.

An unspecified number of SDF fighters were seen moving south from their stronghold of Tel Abyad near the Turkish border toward Ain Issa, a town about 60 km north west of Raqqa, and clashes were reported nearby, according to the Syrian Observatory for Human Rights monitoring group.